The invention relates to a circuit arrangement for commutating a reluctance motor having a rotor rotatable relative to a stator. The stator has a number of windings to which current pulses of predeterminable length and phase relative to the motion coordinate of the rotor are fed based on the position and speed of the rotor.
U.S. Pat. No. 4,739,240 discloses a conventional circuit arrangement for commutating a reluctance motion. The commutator for a microcomputer based switched reluctance drive includes a memory, for example, a ROM, for storing stator phase firing patterns. This commutator enables selective adjustment of the turn-on angle and pulse length of phase switching current pulses. Firing patterns differing only in pulse length of their respective pulses can be stored in the memory. The adjustment of pulse phase and pulse duration allows torque control of the motor over a wide range.
A conventional converter circuit is also disclosed in a book entitled "Brushless Permanent-Magnet and Reluctance Motor Drives" by T. J. E. Miller, Clarendon Press, Oxford, 1989, pp. 177 to 180. More specifically, FIG. 7.16(b) with attendant description on page 178. last paragraph, page 179 as well as page 180, first paragraph, a converter circuit for an n-phase motor in which only n+1 transistors are necessary. For controlling the current amplitude, high-frequency "current chopping" is performed in one "top" transistor of that common circuit to all phases, i.e. windings of the motor. The "bottom" transistors commutate the chopped voltage or chopped current in a precisely predetermined order to the phases of the motor, controlled by a sensing means for determining the position of the motor shaft and by a gate logic. As will be explained hereinafter, this simplified switching circuit will practically not lose on "functionality" compared with a "complete" switching circuit comprising two transistors per phase. However, it should be observed that at high speeds the phases cannot be turned off sufficiently fast by the simplified commutator so that a braking torque occurs as a result of which the difference between driving torque and braking torque--referenced nett torque--diminishes very fast with higher speeds and the motor losses are augmented.
Furthermore, it has appeared that also the measures described in U.S. Pat. No. 4,739,240 are not suitable for remedying the abovedescribed disadvantages at higher speeds of the reluctance motor.
It is an object of the invention to provide a simple circuit arrangement for commutating a reluctance motor, which arrangement ensures even with very high speeds a perfect commutation and thus a high torque and low losses.
This object is achieved by means of a circuit arrangement in which after each supplied current pulse at least the next current pulse in the cyclic train, predetermined for low speeds, is omitted. The next current pulse is omitted when speeds exceed a predeterminable value.
The circuit arrangement according to the invention can be used for operating both rotary and linear motor configurations. With the conventional rotary motor in which the stator and rotor are arranged in a rotationally symmetrical manner, the windings on the stator have been provided in a specific: angle division and order along the circumference and thus the motion coordinates of the rotor have specific, preferably uniform angular spacings. The order of the windings along the motion coordinates represents a first cyclic arrangement in which a cycle is formed by an angle of 360.degree., i.e. one rotation of the rotor. The first cyclic arrangement of the windings is passed after each complete rotation of the rotor.
However, a cycle for a linear motor can also be determined such that it comprises the path or section respectively, of the stator on which the described windings are arranged in a predetermined order. A longer stator would then accordingly be formed by linking a plurality of such cycles.
When the rotor moves relative to the stator along the motion coordinate, the spatial distances covered by the rotor from the beginning of a current pulse to the next current pulse remain constant according to the state of the art. The associated time intervals, however, are shortened when higher speeds are desired. Because the switch-on and off operations of the current pulses in the windings depend on the inductance of the windings and on the available voltages, the time intervals necessary for this switching will essentially remain unchanged even with higher speeds, whereas the time distances between the beginnings of two current pulses become ever shorter with increasing speeds. When a specific speed determined by the structure of the reluctance motor concerned is exceeded, the time-distance between two successive current pulses will fall short of a value below which it is no longer possible to provide current pulses having switch-on and off operations in the required form. In particular successive current pulses start to overlap so that the aforementioned braking torques and stator losses occur.
The invention now eliminates this detrimental effect in a simple manner in that specific sequential current pulses are omitted which are otherwise present in the sequence of current for individual windings provided for at low speeds, i.e. in the cyclic sequence of the windings, that is to say, the windings concerned at this point of the motion coordinates of the rotor are not supplied with current when speeds exceed a predeterminable value that can be determined based on the motor configuration for each case. For example, every second current pulse may be omitted. It is also possible to form only every third current pulse and omit two current pulses between each pair of current pulses formed, etc. In this manner each single current pulse has the disposal of a larger section of the motion coordinate when the time duration is unchanged, without the possibility of two successive current pulses overlapping. Thus it is possible to avoid the damaging braking torque and reach higher speeds with the rotor.
Preferably, the supplied current pulses in excess of the predeterminable speed are lengthened by a predeterminable value relative to the motion coordinate of the rotor such that their final positions remain unchanged relative to the motion coordinate. Consequently, larger time intervals are available for the supplied current. By maintaining the final positions of the current pulses while lengthening the current pulses damaging braking torque is avoided. The sufficient time duration of the current pulses then makes faultless speed and power controls of the reluctance motor possible even at high speeds.
A circuit arrangement according to the invention devised for feeding the reluctance motor from adc voltage source advantageously comprises an asymmetrical H circuit for each winding of the reluctance motor with a first longitudinal branch, common to all the H circuits, and a second longitudinal branch arranged between terminals of the dc voltage source. Each of the second longitudinal branches include a switch element in series combination with a rectifier element. The rectifier element is reversely poled relative to the dc voltage source. In each H circuit the switch element of one longitudinal branch and the rectifier element of the other longitudinal branch are connected to the same terminal of the dc voltage source, and the associated winding in the transverse branch of the H circuit is arranged between the junctions of the switch element and rectifier element of each longitudinal branch.
The circuit arrangement in accordance with the invention is of simple construction and operation even at very high speeds.
Application of the above-described circuit arrangement for purposes of speed or power control, which includes a current control scheme, can be easily achieved by having associated switch elements of the second longitudinal branches coupled through a common measuring element to the dc voltage source terminal of the second longitudinal branches coupled through.
Further to the disclosure of the above-described embodiments of the circuit arrangement according to the invention, reference is expressly made to the contents of DE-OS 38 19 097.
In accordance with a feature of the invention, a position signal generator generates a position signal that represents the position of the rotor along the motion coordinate. The circuit arrangement can also include a speed signal representing the speed of the rotor which is derived from the time-dependent changes of the position signal. A separate sensor for sensing the speed of the rotor will then not be necessary.
The circuit arrangement in accordance with the invention involves relatively little circuitry little circuitry at low cost by requiring that the rotor be subdivided in to a second cyclic arrangement. The position signal generator preferably comprises a motion coordinate generator connected to the rotor, subdivided in accordance with the second cyclic arrangement having a number of sensing elements. The number of sensing elements corresponds to the second cyclic arrangement. The sensing elements are interspaced along the motion coordinate of the rotor by a unit of length that forms a part of a complete cycle of the stator arrangement determined by the least common multiple of the first and second cyclic arrangements. The motion coordinate generator has a cyclic arrangement such that the sensing elements produce a first signal value over a first section of two units of length and a second signal value over a contiguous second section of four units of length.
A conventional signal generator such as described in British Patent Specification 1 572 586 includes a disk drive system having a brushless dc motor. The motor has a floppy disk rotatable by a rotor. The disk has a circular array of transparent slots or apertures distributed around its periphery forming a tachometer track. This tachometer track is scanned with an optical sensing assembly which produces a train of timing signals when the floppy disk is rotating. These timing signals are processed as speed signals in a speed control network. The floppy disk also has a pair of diametrically opposite radial extensions or paddles which are opaque and occupy a 90.degree. sector on the disk perimeter. Opposite these paddles are three optical sensing assemblies evenly distributed over a 60.degree. sector of the disk interspaced by 30.degree.. Each of these sensing assemblies produces an output signal only if neither paddle is positioned opposite that assembly. When the floppy disk rotates, three square-wave position signals are produced, with the pulses in the signals being displaced in phase by an angle of 30.degree..
In contrast thereto, the position signal generator in accordance with the invention is used for forming both the position signal and the speed signal which leads to aforementioned simplification in construction and operation of circuitry for commutating a reluctance motor.
A highly cost-effective and compact structure, adaptable to different commutating reluctance motor structures, the circuit arrangement according to the invention further includes a control array responsive to the position signal and speed signal, for switching the current pulses applied to the windings. The control array preferably comprises a memory arrangement for storing switch information signals corresponding to the various combinations of position and speed signals. The memory arrangement can include different types of ROM configurations.
Unlike the control system of the present invention, conventional control systems for a brushless dc motor such as described in U.S. Pat. No. 4,479, 078 include ROM configurations for controlling the corn mutated energy supplied to the windings arrangements. Also for the reluctance motor and the operational mode according to the present invention a simplification and price reduction are provided.
Further advantageous embodiments of the invention are claimed in the dependent claims.